Arthas is holding back the Scourge from annihilating all of us. At first this makes no sense. He cut out his hard and it was destroyed. Tirion said there's nothing but a monster left. How could Arthas be protecting us if he has no good left in him?

Arthas the Lich King is not. He's trying to kill us. But Frostmourne on the other hand...

Arthas, the brave paladin who would sacrifice anything, even his own soul to save his people, he had his soul taken by Frostmourne. He's in there. And along with all the other souls, they may be able to restrain the Lich King.

This may also explain why Arthas would leave Frostmourne somewhere else. He may know that the blade is holding him back. But he cannot reject it or let it fall into other hands, for then he'd lose power. So he leaves it in the Halls of Reflection. As an added benefit, it acts as bait, luring in irritants like Jaina and Sylvanas where they can be destroyed or enslaved.

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I hadn't really thought about it that way. Even though I did the three new 5-mans, I didn't get to enjoy them very much because I felt like I was making everyone wait. :( I was the only newbie in my group.

Frosty the swordman was a lot of captured souls,With a cornucopia of demon magicAnd two eyes made out of soul.Frosty the swordman is a fairy scary tale,He was made of metal but the LegionKnow how he came to life one day.There must have been some magic in thatOld shaman they found.For when they placed him in his throneHe began to hunt around.O, Frosty the swordmanWas undead as he could be,And the acolytes say he could huntAnd kill just the same as you and me.Thumpetty thump thump,Thumpety thump thump,Look at Frosty go.Thumpetty thump thump,Thumpety thump thump,Over the Icecrown snow.

Frosty the swordman knewThe Light was hot that day,So he said, "Let's run andWe'll have some funNow before I melt away."Down to the village,With a Frostmourne in his hand,Running here and there allAround the square saying,Escape me if you can.He led them down the streets of townRight to the necromancer.And he only paused a moment whenHe heard him holler "Rise!"For Frosty the swordmanHad to hurry on his way,But he waved goodbye saying,"Don't you cry,I'll be back again some day."Thumpetty thump thump,Thumpety thump thump,Look at Frosty go.Thumpetty thump thump,Thumpety thump thump,Over the Icecrown snow.

@Ateve: Can you source the original lore on the armor? That's new to me.

@jeffo: Fortunately, I think Tirion is too pure to become evil. We could potentially have a good Lich King. In that case I imagine he'd free the sentients which he could (second Forsaken) and maybe the rest would be used as labor to help rebuild the world. He'd probably slaughter the free members, such as the Cult of the Damned and all the necromancers.